(a) Field of the Invention
White biaxially oriented polypropylene films have traditionally been produced using a cavitated/expanded core in order to achieve opacity. The cavitated/expanded core weakens the core which is generally the thickest layer of such films.
In the invention of this application, a thin polyolefinic skin layer containing titanium dioxide is used to form a cold seal receptive layer which, in cooperation with the metallized surface on the opposite side of the core layer, enhances the white coloration of the titanium dioxide containing layer. Such films have better physical properties, e.g., tensile strengths, since the core has not been modified for opaqueness.
The cold seal receptive polyolefinic layer is on one side of the core and preferably a polyolefinic skin layer which is subsequently metallized is on the other side of the core. Each of the skin layers has a thickness of less than one-fifth of that of the core. Thus, the core can contain a large percentage of a high tensile strength polyolefin such as isotactic polypropylene homopolymer or high density polyethylene homopolymer whereas the skin layers are thin and provide for a film which has high tensile strength for its overall thickness.
(b) Discussion of the Prior Art
1. U.S. Pat. No. 4,230,767 of 10-28-80 to T. Isaka, et al. discloses an oriented heat sealable film of a polypropylene polymer base layer and a surface layer or layers of a copolymer of ethylene and propylene and a copolymer of butene and any other polymerizable monomer having ethylenic unsaturation. One surface of the film can be subjected to electro-discharge treatment. Slip agents and antiblocking agents can be added to the film.
2. U.S. Pat. No. 4,357,383 of 11-02-82 to M. Howden et al. discloses a multi-layer metallized film of an alpha-olefin having on at least one surface an adherent layer of random copolymer of ethylene with 0.5 to 15% of 3-6 carbon alpha-olefin and a metallic layer on the surface of the adherent layer remote from the substrate. Among a number of options, this film can also have a heat seal layer on the surface opposite from the metallized layer.
3. U.S. Pat. No. 4,692,380 of 09-08-87 to D. Reid discloses a metallized, biaxially oriented polypropylene film which exhibits good adhesion between the polypropylene and the metal coating. The core has on at least one surface a propylene-ethylene copolymer which contains no slip agent and which has been subjected to corona treatment.
4. U.S. Pat. No. 4,741,950 of 05-03-88 to L. Liu et al. discloses an oriented, opaque, alpha-olefin laminate film having a non-blocking first surface, a smooth lustrous second surface which is intended for further film processing operations such as metallization and an expanded core to provide opacity. The core can optionally also contain additional filler for opacity and the anti-blocking skin layer optionally contains 2% to 8% of titanium dioxide to confer enhanced opacity. Among other shortcomings this patent uses voids in the core which adversely affect the mechanical properties and the anti-blocking layer is not heat sealable.
5. U.S. Pat. No. 4,758,396 of 07-19-88 to B. Crass et al. discloses biaxially oriented film having a single ply or an opaque core layer of a multiply film composed essentially of polypropylene. The opaque core layer includes 10-40% of filler. Titanium dioxide is mentioned as one of the fillers. In addition to the polypropylene opaque single ply or multiply layered film such film can include functional layers such as heat sealing, cold sealing, adhesion promoting, dye coating, and metallizable layers. The film of this prior patent has a density of 0.6 grams per cubic centimeter.
6. U.S. Pat. No. 4,997,700 of 03-05-91 to L. Bothe et al. discloses a metallizable, heat sealable, biaxially oriented multilayer film which comprises a polypropylene core layer, a first polyolefin heat-sealable surface layer and a second polyolefin metallizable surface layer. The metallizable layer comprises a propylene-ethylene copolymer containing from 1.2 to 2.8 % of ethylene. The heat sealable layer comprises ethylene containing polymers, copolymers of ethylene and propylene or copolymers of propylene and 1-butane as well as other olefinic mixtures. In contrast to densities of about 0.85 to 9.5 grams per cubic centimeter for the films of this invention, the density of the U.S. Pat. No. 4,997,700 patent is recited as 0.8 grams per cubic centimeter or less, particularly 0.65 grams per cubic centimeter or less.
7. U.S. Pat. No. 5,026,592 of 06-25-91 to S. Janocha et al. discloses an opaque, biaxially stretched oriented, polyolefinic multilayered film comprising an opaque core layer and two transparent skin layers. The opaqueness is attained by the use of inorganic fillers in the core layer. Titanium dioxide is mentioned along with many different fillers as suitable. Among many different options the top layers can be sealable or non-sealable and metallized or non-metallized.
8. U.S. Pat. No. 5,091,236 of 02-25-92 to L. Keller et al. discloses a multilayer opaque, biaxially oriented polymeric film. The film structure includes: (a) a core layer having voids which cause a significant degree of opacity; (b) a layer adhering to the core Layer including up to about. 12% of titanium dioxide: and a titanium dioxide-free non-voided thermoplastic skin layer adhering to the other side of the core layer. The titanium dioxide improves opacity and whiteness of the film.
9. HERCULES WTF 503 is a non-sealable film having a white-opaque oriented polypropylene core and a metallizable surface designed for the insulation market. This product is sold by Hercules, Inc.
The present invention has the following main advantages over the prior art:
1. A significant white-opaque appearance is achieved by cooperation of the metallized layer with the layer containing the titanium dioxide without the need for cavitation, expansion, or inert filling of the core layer. This also eliminates the need for white color printing of the film. This provides for the use of smaller quantities of filler to attain the white-opaque look as compared to fillers in the core, provides better mechanical properties, and is more economical. PA1 2. The white pigmented layer provides excellent cold seal receptivity and adhesion in spite of the relatively large percentage concentration of filler in this thin layer. PA1 3. The excellent mechanical properties such as high stiffness and tensile strength of conventional polyolefin films, e.g., biaxially oriented isotactic polypropylene homopolymer film, are maintained versus cavitated, expanded, or filled opaque polyolefin film. PA1 4. Thinner gauges (higher yields) of laminated film can be used versus cavitated/expanded, opaque alpha-olefin film thus providing a cost advantage. PA1 5. Excellent metal bond strength as well as excellent gas barrier properties are obtained.
The films of this invention are particularly useful as wrappers or packages for confectionery and baked items, e.g., candy bars, cup cakes, and frozen confections such as ice cream bars. The consuming public appears to associate the white coloration with sanitation and such white coloration helps mask oil and grease from the confectionery or baked items.